In recent years, the range of applications for magnetic recording devices, such as a magnetic disk device, a flexible disk device, and a magnetic tape device, has widened significantly and the importance thereof has also increased. In addition, the recording density of the magnetic recording media used for these devices has increased significantly. In particular, with the introduction of an MR head and a PRML technique, the surface recording density has increased sharply. In addition, in recent years, with the introduction of, for example, a GMR head and a TMR head, the surface recording density has increased at a rate of about 100% per year. There is a demand for magnetic recording media with a higher recording density. In order to meet the demand, it is necessary to achieve an increase in the coercivity of the magnetic layer, a high signal-to-noise ratio (SNR), and a high resolution. In addition, in recent years, there is an attempt to increase the surface recording density with an increase in the line recording density and in the track density.
The latest magnetic recording device has a maximum track density of 110 kTPI. However, when the track density is increased, the magnetic recording information items of adjacent tracks interfere with each other, and a magnetization transition region, which is a boundary region between adjacent tracks, serves as a noise source, which causes a reduction in SNR. The reduction in SNR leads to a reduction in a bit error rate, which is an obstacle to the improvement of the recording density. In order to increase the surface recording density, it is necessary to reduce the size of each recording bit on the magnetic recording medium and ensure the largest possible saturation magnetization for each recording bit and the largest possible thickness of a magnetic film. However, when the size of the recording bit is reduced, the minimum volume of magnetization per bit is reduced, and recording data may be lost due to magnetization reversal caused by heat fluctuation.
Since the distance between the tracks is short, the magnetic recording device requires a very accurate track servo technique, and a method is generally used which performs recording over a wide range and performs reproduction in a range narrower than that when recording is performed in order to minimize the influence of adjacent tracks. In this method, it is possible to minimize the influence of adjacent tracks, but it is difficult to obtain an adequate reproduction output. Therefore, it is difficult to ensure a sufficient SNR.
As one of the methods for solving the problem of the heat fluctuation and ensuring an adequate SNR or an adequate output, there is an attempt to form concave and convex portions on the surface of a recording medium along the tracks to physically or magnetically separate the recording tracks, thereby improving the track density. This technique is called a discrete track method and a magnetic recording medium manufactured by the method is called a discrete track medium in the following description.
As an example of the discrete track medium, a magnetic recording medium has been known which is formed on a disk-shaped substrate having an uneven pattern formed on the surface thereof, thereby forming magnetic recording tracks and servo signal patterns that are physically separated (for example, see Patent Document 1).
In order to manufacture the discrete track medium, any of the following methods are used: a method of forming a continuous magnetic recording layer including a predetermined number of thin films and physically processing the magnetic recording layer to form tracks; and a method of forming an uneven pattern on the surface of a substrate in advance and forming a thin film of a magnetic recording medium (for example, see Patent Documents 2 and 3). Of the two methods, the former is often called a magnetic layer processing type. The latter is often called an embossing type.
In addition, a method has been proposed which injects nitrogen ions or oxygen ions into a previously formed magnetic layer or emits a laser beam to the magnetic layer, thereby forming a region between the magnetic tracks of the discrete track medium (see Patent Document 4).
There are a glide inspection process and a certified inspection process as a total inspection process for the magnetic recording medium manufactured by the above-mentioned method.
The glide inspection process examines whether there is a protrusion on the surface of the magnetic recording medium. That is, during the recording or reproduction of the magnetic recording medium by the magnetic head, when there is a protrusion with a height equal to or greater than the gap between the medium and the magnetic head on the surface of the medium, the magnetic head collides with the protrusion and is damaged, which causes a defect in the medium. This process examines whether there is a high protrusion (for example, see Patent Document 5). This process examines the presence of a protrusion on the surface of the magnetic recording medium and does not record or reproduce signals on or from the magnetic recording medium.
The certified inspection process is performed on the magnetic recording medium which has passed the glide inspection process. The certified inspection process records a predetermined signal on the magnetic recording medium using the magnetic head as in a general recording/reproducing process of the hard disk drive, reproduces the signal, and checks the quality of the medium, such as electrical characteristics or defects, from the obtained reproduced signal (for example, see Patent Document 6).
[Patent Document 1] JP-A-2004-164692
[Patent Document 2] JP-A-2004-178793
[Patent Document 3] JP-A-2004-178794
[Patent Document 4] JP-A-5-205257
[Patent Document 5] JP-A-10-105908
[Patent Document 6] JP-A-2003-257016